1. Field of the Invention
The present invention relates to an optoelectronic semiconductor device and a light signal input/output device.
2. Background Art
In recent years, optical interconnection technology has been seen as a leading technology for long-distance, large-capacity signal transmission. Optical interconnection technology uses light to transmit signals within an electronic device of a system and between electronic devices. For example, an optical subscriber line in an FTTH (Fiber To The Home) system is one kind of optical interconnection technology in a broad sense, as an optical fiber is used to transmit information from a center station to each subscriber.
Electrical signal transmission is known as one way of transmitting signals at a high speed. In order to improve the performance of electronic signal transmission modules, much research and development has been conducted on improving the performance of silicon LSIs. As a result, the operation speed and the integration scale of LSIs have been improved remarkably. However, with respect to the improvement of the performance of electronic signal transmission modules, there are still problems to be solved, i.e., the improvement of the transmission rate and the wiring density of electrical signal lines. Specifically, even if the performance of transistors and other functional elements in an LSI is improved, a lack of improvement of the transmission rate and the wiring density of electrical signal lines would serve as a rate limiting factor, and prevent the improvement in performance of electronic signal transmission modules. Furthermore, signal transmission delays in electrical signal transmission lines also serve as a limiting factor for the improvement in performance of electronic signal transmission modules. Even if a higher signal transmission rate and a higher wiring density of electrical boards can be achieved, the influence of EMI (Electromagnetic Interference) is still notable. For the aforementioned reasons, optical interconnection technology has been attracting attention as a technology for solving the above problems of the electronic signal transmission modules.
In optical interconnection technology, a light signal input/output device receives a light signal, performs a redetermined processing operation on the light signal, and then retransmits the processed light signal. In the light signal input/output device, an optoelectronic semiconductor device is provided on an electro-optical wiring board having electric wires and optical waveguides formed therein. The optoelectronic semiconductor device includes a photoreceptor or photodetector element, which receives a light signal from the optical waveguides and converts the light signal into an electronic signal; a semiconductor integrated circuit element, which processes the electronic signal from the photoreceptor; and a light emitting element, which converts the electronic signal from the semiconductor integrated circuit element into a light signal and retransmits the light signal to the optical waveguide. The light signal input/output device is disclosed in, for example, Japanese Patent Publication No. 2000-332301.
Optoelectronic semiconductor devices are becoming smaller and denser, and optoelectronic semiconductor devices of about 10 mm×10 mm in size are common. In order to effectively perform the miniaturization and the increase in density, the BGA (Ball Grid Array) mounting technique is seen as an effective technique for mounting an optoelectronic semiconductor device on an electro-optical wiring board. A BGA has a structure in which solder ball electrodes are arranged to form a grid. The BGA structure is smaller than that of a QFP (Quad Flat Package) using outer-lead electrode terminals since the solder ball electrodes occupy less space. Furthermore, the BGA structure incurs no such deformation of lead as will occur frequently with the QFP. Moreover, the surface tension of the solder balls being molten causes a self-alignment effect which automatically corrects a misalignment having occurred during the mounting of the optoelectronic semiconductor device. An optoelectronic semiconductor device employing the BGA structure is disclosed in Japanese Patent Publication No. 2001-185752, for example.
As optical interconnection technology is likely to become a higher density and higher speed communication technology for the next generation, it is essential to ensure a highly reliable connection between the optoelectronic semiconductor device and the electro-optical wiring board, and to accurately position them in relation to each other. For achieving such connection and positioning, the above-mentioned BGA mounting technique is particularly promising.
In a conventional optoelectronic semiconductor device using the BGA mounting technique, a cavity is formed in the lower of upper and lower sides of a circuit wiring board where solder ball electrodes are formed, and an optoelectronic semiconductor element and a semiconductor integrated circuit element are placed in the cavity so that the optoelectronic semiconductor element and the electro-optical wring board may face each other, as disclosed in Japanese Patent Publication No. 2001-185752. This method (cavity-down BGA) is effective for the improvement of the heat radiation characteristic as well as for miniaturizing the device. However, since the manufacturing cost of the circuit wiring board having the cavity is high, the packaging cost of the semiconductor device is also high.
There are optoelectronic semiconductor devices in which major consideration is given to a decrease in the manufacturing cost. In such a case, circuit elements can be provided on the upper side of a circuit wiring board while solder ball electrodes can be provided on the lower side of the circuit wiring board. In such an optoelectronic semiconductor device, it is necessary to form light input/output through-holes in the circuit wiring board in order to transfer a light signal between an optoelectronic semiconductor element and an electro-optical wiring board. However, it is considered extremely difficult to actually use this technique. That is, in optical interconnection technology, high density transmission is performed with an increased number of light signals. Accordingly, an increased number of light signals are used in the optoelectronic semiconductor device. Thus, the optoelectronic semiconductor device has many optoelectronic semiconductor elements provided therein. When circuit elements are provided on the upper side of the circuit wiring board, however, since they are disposed in the form of a one-dimensional array on the periphery of a semiconductor integrated circuit element, the size of the circuit wiring board increases. When the circuit wiring board becomes larger, a stress-induced strain caused by the difference in the coefficient of thermal expansion (CTE) between the circuit wiring board and the electro-optical wiring board also becomes larger. In addition, since the light input/output through-holes are formed in the circuit wiring board as described above, the strength of the circuit wiring board is decreased. Therefore, it has been believed that if the circuit elements are provided on the upper side of the circuit wiring board, a stress-induced strain would cause damage to the circuit wiring board, and/or would peel off the solder ball electrodes, thereby decreasing the reliability of the device.